My question deals with documented vessels purchased in Delaware and Documented. If the vessel is kept in Delaware for more than 183 days is it subject to the Maryland Excise tax if it sails in Maryland waters for 150 days?

Excise tax...probably yes (I live in New England...).

But you should be asking about the sales/use tax, as typically that's the big one. And the general answer is...you will pay the sales/use tax in a state where you have the boat, less whatever you paid in a different state. The whole tread above considers some of the ifs and buts in actual PA/MD practices.

One clarification, if while a resident of a state with a sales/use tax you buy a boat in a different state, and pay sales/use tax to that different state, be extra careful to retain legal proof of the boat's "non-residency". Should you subsequently bring the boat to your residency state, that state may demand their sales/use tax, with no credit for the previous payment, unless you can prove the "non-residency" of the boat at the time you paid the other tax.